Liquid crystal film on gold produces ultra-compact color filter

Schematic of the tunable color filter. The combination of a gold film with ring-shaped holes and the use of liquid crystals (red and green) enables pixels of a defined color that can be turned on and off. Credit: 2012 Y. J. Liu

Flat
panel displays, mobile phones and many digital devices require thin,
efficient and low-cost light-emitters for applications. The pixels that
make up the different colors on the display are typically wired to
complex electronic circuits that control their operation. Jing Hua Teng
at the A*STAR Institute of Materials Research and Engineering and
co-workers have now developed a display technology that requires a much
simpler architecture for operation. They demonstrated that combining a
thin perforated gold film with a liquid crystal layer is all that it
takes to make an efficient color filter.

“Our
color filters are a lot thinner and more compact than conventional
thin-film-based color filters,” says Teng. “The colors of these filters
can be tuned with ease so they are very versatile in applications.”

The
color selection of the devices comes from the patterned gold film. The
collective motions of the electrons on the film surface—the so-called
surface plasmons—absorb light at wavelengths that depend on the details
of these patterns. In the present case, the patterns are narrow,
nanometer-sized rings cut out of the films (see image). As the diameter
of the rings changes, so does the color of the metal film. Pixels of a
different color can be realized simply by patterning rings of different
sizes across the same gold film.

To
realize a full display, however, each of these pixels needs to be
turned on and off individually. This is where liquid crystals come in.

Liquid
crystals are molecules that can be switched between two different
states by external stimuli, such as ultraviolet light. In their normal
state the crystals let visible light pass through so that the pixel is
turned on. But when ultraviolet is also present, the structure of the
liquid crystal molecules will change so that it absorbs visible light
(i.e. the pixel is turned off). This process can be repeated over many
cycles without degrading the device itself.

Although
the device works in principle, it remains a concept on the drawing
board for now. This is because there are still many issues that need to
be overcome, for example, the optimization of the switching speed and
the contrast between ‘on’ and ‘off’ states. In future work, the
researchers will need to extend their ideas so that their device can
serve a larger area and produce the fundamental colors red, green and
blue.

Teng and his team are quite optimistic that they will achieve this soon.